The present invention relates to telecommunication line conditioning devices. More particularly, the invention relates to an electronically and remotely controllable slope equalizer for wireline signal transmission, such as for twisted pair local loops.
1. The Local Loop
In telephony systems, telephone subscriber equipment, such as telephones and modems, are connected to the telecommunication network by way of twisted pairs of copper wires. The wire connection from the telephone network to the customer premise is commonly referred to as the local loop. The local loops from numerous locations terminate at a xe2x80x9ccentral officexe2x80x9d (xe2x80x9cCOxe2x80x9d) of the local telecommunication provider or at remote telecommunication pedestals. Channel banks, within the CO, such as exemplary D4 channel banks, convert analog signals from a plurality of loops into a digital form.
Various impairments on the local loop can interfere with communication. One source of impairment, the frequency response characteristic of the local loop, among other things, is a function of the length of the wiring connecting the subscriber to the channel card. That is, as the cable length increases, the varying amounts of attenuation at various frequencies can distort the local loop signals. Typical local loop wiring can range from a few hundred feet, or up to five or six miles, depending on the proximity of the customer premises to the CO.
Typically the attenuation caused by the cable increases with frequency, resulting in significantly more attenuation at higher frequencies. This causes the frequency response to exhibit a xe2x80x9cslopexe2x80x9d (as typically graphically illustrated as gain versus frequency, measured as the dB difference between two reference frequencies, typically 1004 Hz and 2804 Hz). Some local loops include coils placed along the wires that improve the frequency response characteristic by flattening out this slope. These lines are referred to as xe2x80x9cloadedxe2x80x9d cables. Even loaded cables, however, can exhibit frequency characteristics with unacceptable amounts of slope. To correct for the slope in the frequency response, slope equalizers are often used.
2. Slope Equalizers
Slope equalizers are used to compensate for the additional attenuation at higher frequencies. By providing varying amounts of gain (or attenuation) with a slope opposite to the frequency response of the cable or transmission line, the slope equalizer may compensate for the frequency response of the local loop.
Variations in the loop characteristics, caused by varying cable lengths and qualities, the presence of loading coils, other line conditions, and so on, make it desirable to have adjustable slope settings for the slope equalizers, to accommodate the characteristics of any given individual loop. Optimal slope settings can be determined from directly measuring the line characteristics. Alternatively, these settings can be prescribed based on features such as whether the cable is loaded or unloaded, the cable gauge, and the cable length.
Local loop slope equalizer settings have been standardized. FIG. 1 is a block diagram illustrating a prior art, manually adjustable slope equalizer 100. The prior art manually adjustable slope equalizer 100 utilizes an operational amplifier (xe2x80x9cop ampxe2x80x9d) 150, having a switch and resistor arrangement 145 fed back into one input, to form a variable-gain portion, with the second input having an input signal VIN, followed by a high-pass filter (HPF) 160 and low-pass filter (LPF) 170. The switch and resistor arrangement 145, more particularly, consists of segments of parallel switch and resistor arrangements, with each parallel segment coupled to the others in series (and referred to herein as a series arrangement). The equalization slope is manually set using four designated and standardized switches, referred to in the art as SW1 (105), SW2 (110), SW4 (115), and SW8 (120). The slope setting, k, is determined by the sum of the index numbers on the open switches SWi. As an example, a slope setting of six is achieved when SW1 and SW8 are shorted or closed, and SW2 and SW4 are open, resulting in a short across the R1 (125) (787xcexa9) and R8 (140) (6.19 Kxcexa9) resistors and open switches across the R2 (130) (1.58 Kxcexa9) and R4 (135) (3.16 Kxcexa9) resistors.
The variable slope portion of the equalizer is typically followed by a high-pass filter 160 and a low-pass filter 170. In the example shown in FIG. 1, the 0.326 uF and 1.96 Kxcexa9 resistor form a high-pass filter 160 with a 3 dB high-pass corner frequency at 249 Hz. This provides a modest amount of additional slope that tends to flatten out the low frequency response of the channel by bringing it down slightly. The low-pass filter 170 of the manual, prior art equalizer of FIG. 1 is typically an RC filter. An additional operational amplifier is preferably added in the low-pass filter for isolation.
The switches SW1, SW2, SW4, and SW8 are typically manually configurable switches, such as DIP switches. Manual switches, however, have the disadvantage of requiring a technician to set the slope values by hand. This increases costs of configuring the slope equalizers, considering the many thousands of line cards in a typical central office. Relays or transistors may be used for the switches shown in FIG. 1, thereby providing the added feature of electronic control (which also might be accomplished remotely), but having inherent disadvantages. Mechanical relays, for example, have a relatively large size, high cost, and are susceptible to failure due to the mechanical nature of the devices.
Transistors are smaller, cheaper, and more reliable than relays, but also have significant disadvantages, particularly with regard to introduced distortion and errors due to their non-negligible on-resistance. Inexpensive transistors such as Field Effect Transistors (such as FETs of MOSFETs) have a relatively high on-resistance; as a consequence, when they are in the conductive mode (i.e., the switch is xe2x80x9conxe2x80x9d), the resistance across the transistor is non-zero and thereby contributes to the overall feedback resistance. This significant on-resistance introduces error, affecting the desired equalization slope, and resulting in inaccurate settings.
With reference to FIG. 1, as an example, consider the case of slope=1, where all switches except SW1 (105) are closed. The on-resistance of the remaining three switches appears in series with the R1 (125), the 787xcexa9 resistor. When the four switches 105, 110, 115 and 120 are implemented as a typical low-cost FET transistor package, such as an 74HC4316 with four FET switches, each switch has a rated, worst case on-resistance of about 170xcexa9. Three of these resistors in series with 787xcexa9 causes an effective feedback resistor of 787+3(170)=1297xcexa9, effectively resulting in the equalizer having an approximate slope=2, rather than the desired slope of 1, introducing a significant error in the frequency response.
In addition, even if the error in the effective resistor value were reduced (by, for example, modifying the scale of the component values throughout the circuit), the variation in the FET""s on-resistance as a function of signal voltage would still cause unacceptable amounts of non-linear distortion. While alternative transistor switches for use as SW1-SW8 may be used that will mitigate to some extent the problems discussed above, however, they are considerably more expensive and may be commercially impractical.
As a consequence, a need remains for an improved slope equalizer structure that may be configured electronically and remotely, rather than manually, that has variable slope settings, that has significantly lower distortion and is more accurate than analog switches would be in conventional slope equalizers, and that may be implemented in a small space, in a commercially reasonable manner and at low cost. In addition, a need remains for a slope equalizer structure which accommodates the need to remotely configure the slope settings, such as through programmable switches, and which provides backwards compatibility with standardized slope equalizer settings.
The present invention provides an improved slope equalizer for use in a voice frequency channel card. The slope equalizer of the present invention includes a high-pass filter stage, followed by a variable gain stage amplifier. In the preferred embodiment, the variable gain stage amplifier is an operational amplifier (op amp (A2)) having first and second inputs; a plurality of parallel input signal paths are provided for the first input of op amp A2, with each of the parallel signal paths being a series-connected resistor and switch, and with the second input of op amp A2 coupled to ground, to provide a virtual ground at the first input of op amp A2. In the preferred embodiment, each of the series switch and resistor combinations are selected to provide retrocompatible programming with standard, prior art equalization configurations.
In parallel with the variable gain stage and high-pass filter stage, in the preferred embodiment, is a unity gain path. The combined output of the variable gain stage and the unity gain stage are input into a summer circuit, which combines the two paths. Preferably, the summer circuit also provides low-pass filtering, eliminating the need for the additional stages of the prior art.
Each of the switches is preferably a FET transistor that is electronically and, optionally, remotely controllable. The circuit topology allows the use of FET transistors having relatively high on-resistances. The on-resistance of each FET switch may be accurately compensated by adjustment of the corresponding series resistor. Once this adjustment is made, all undesired switch interaction based upon on-resistance is removed. A further advantage of the topology is that the FET switches are connected to a virtual ground by way of an operational amplifier, thereby reducing on-resistance modulation or other distortion caused by signal variations across the series-connected resistor-switch combination.
As a consequence, the slope equalizer structure of the present invention may be configured electronically and remotely or locally, rather than just locally. The slope equalizer of the present invention has electronically configurable slope settings using small, inexpensive, easily integrated components. Further, the topology is beneficially arranged to minimize any non-ideal effects of switch on-resistance, as well as minimizing any distortion due to modulation of on-resistance with signal.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.